1. Field of the Invention
This invention relates to silica xerogels having physical characteristics making them useful as catalyst supports and, more particularly, to xerogels having cumulative pore volumes, pore diameter distributors, and surface areas which uniquely qualify such materials as supports for stereospecific catalysts for particle form olefin polymerization reactions in which relatively low molecular weight, high melt index polyolefins are produced.
2. The Prior Art
Silica gels find numerous applications, chief amongst which are as adsorbents and catalyst supports. The latter application in particular has attracted increasing attention in recent years, especially in connection with catalysts for the stereospecific polymerization of olefins. Catalysts having stereospecific activity include metal-containing catalytic materials, e.g. chromium oxides, deposited on silica or, alternatively alumina, supports which have previously been activated by oxidation at elevated temperatures. Olefins may be polymerized with such catalysts to produce a varied series of polymers having differing molecular weights and melt indexes, depending upon the particular temperatures, pressures, solvents or other diluents, catalysts, or other reaction conditions used.
For many applications the production of low molecular weight, high melt index polymers is of particular advantage, such materials finding important applications in films and sheets, extrusion coating, injection and rotational molding, and the like. Considering the preparation of ethylene polymers as illustrative, low molecular weight (high melt index) polyethylenes are commercially obtained by carrying out the polymerization in solution (the "solution process"), but only with conversions of less than about 1,000 pounds of polyethylene per pound of supported catalyst (equivalent to .gtoreq. 10 ppm as Cr on a polymer basis as Cr content of supported catalyst is .gtoreq. 1%). On the other hand, when the reaction is carried out in suspension (The "suspension" or "particle form" process), it is possible to obtain conversions of from about 5,000 to 15,000 pounds of polyethylene per pound of supported catalyst having Cr content of .ltoreq. 1% (equivalent to .ltoreq. 2 ppm Cr on a polymer basis). Moreover, it is necessary in the solution process, in order to preserve the color and desired appearance of the product resin, to maintain the chromium content in the resin lower than about 2.5 ppm. The catalyst must, therefore, be removed from the polymer product formed in the solution process. The catalyst need not, however, be so removed during particle form processing. The particle form process thus exhibits distinct commercial advantages relative to the solution process for the stereospecific polymerization of olefins.
Heretofore, however, particle form or slurry operations have been limited, at high conversion rates equal to or greater than about 5,000 pounds of polyethylene per pound of catalyst, to the production of polyolefins having melt indexes lower than about 2. Various techniques have been proposed to increase the melt indexes of olefin polymers so produced, with varying degrees of success. For example, while the use of modifiers such as hydrogen has been found to decrease the molecular weight and increase the melt index of the polymer product, the advantages attendant the use of such materials are limited since they simultaneouly decrease catalyst activity. Similarly, variation of the chromium oxide content of the catalyst, addition of different metal oxide promoters, combination of different supports and/or the use of varying catalyst activation temperatures, have been widely investigated, with only marginal improvement.
Modification of the porosity, surface area and other characteristics of the catalyst support has also been suggested as a means for increasing the melt index of olefin polymers produced by particle form stereospecific polymerization reactions. Thus, in recent years a number of procedures have been described in the literature for the preparation of silica gel materials said to be useful as catalyst supports for this purpose. Such procedures are described, for example, in U.S. Pat. Nos. 3,132,125 and 3,225,023; and in British Pat. No. 1,007,722. Silica gels so prepared have not, however, achieved their intended purpose, i.e., the production of olefin polymers having markedly increased melt indexes.
Thus, for example, Schwander et al U.S. Pat. No. 3,132,125 describes the use in both solution and suspension processes of stereospecific catalysts supported on non-porous silicas for the production of polyolefins said to have relatively low average molecular weights and, correspondingly, high melt indexes. Relatively high melt index polymers were in fact produced in the solution phase operations exemplified by Schwander et al. Where, however, particle form operations were utilized use of the catalyst described in this patent resulted in the preparation of polymer products having melt indexes (estimated from the molecular weight data set forth by Schwander et al) no greater than about 0.2.
Hogan et al U.S. Pat. No. 3,225,023, assigned to Phillips Petroleum Company, suggests that olefin polymers having increased melt indexes may be produced employing catalyst supports having increased average pore diameters, ranging from about 60 to 400 A. Hogan et al illustrate their process by experimental runs (which may have been conducted in either the solution or suspension phases), employing "commercial silica gel" supports having varying average pore diameters. The use of silica gels of the type commercially available as of the Hogan et al filing date (November, 1962) and having the indicated range of average pore diameters has not, however, resulted in the formation of very high melt index polymers employing particle form operations. Thus, polyethylenes so produced (Employing chromium oxide catalysts deposited on such supports) have melt indexes of only up to about 3.0.
British Pat. No. 1,007,722, also assigned to Phillips Petroleum Company, describes the use of "a specific form of high purity finely divided porous silica gel" as a support for a chromium oxide catalyst said to be capable of producing relatively high melt index polyethylenes in a particle form polymerization. The specific form of silica gel referred to in the British specification is a silica aerogel having a pore diameter between approximately 200 A and 500 A, a surface area of approximately 250 to 350 m.sup.2 /g, a density of less than approximately 0.2 g/ml., and an oil adsorption of approximately 300 lbs/100 lbs. "Syloid" 244 (having a surface area of 250 m.sup.2 /g, a pore volume of 2.2 cc/g, and a pore diameter of 350 A) is the sole such material exemplified.
Aerogels are silica gels in which the liquid phase has been replaced by a gaseous phase in such a way as to avoid shrinkage as occurs by direct evaporation of the liquid phase thereof (materials prepared in the latter manner being termed xerogels); Iler, The Colloid Chemistry of Silica and Silicates, Cornell University Press, pages 137 and 152. Aerogels are, however, subject to subsequent shrinkage when wetted due to coalescence of their ultimate particles. Shrinkage of this nature decreases porosity and markedly impairs the use of these materials as stereospecific catalyst supports. Moreover, aerogels readily disintegrate when subjected to mechanical stress. Thus, it has been found that the use of silica aerogels as catalyst supports in the particle form process is less than satisfactory.
Nor have other recently disclosed silica gel materials having varying porosity and surface area characteristics proved adequate to effect the production of high melt index olefin polymers in particle form operations. Such materials are disclosed, for example, in U.S. Pat. Nos. 2,731,326; 3,403,109; 3,428,425; and 3,669,624; and in British Pat. No. 1,077,908.
As illustrative, Hyde U.S. Pat. No. 3,453,077, and British Pat. No. 1,077,908, both of which are assigned to W. R. Grace and Co., disclose methods said to result in the preparation of "microspheroidal silica gels" having pore volumes within the range of from as low as 0.3 cc/g (the British specification) to as much as 2.5 cc/g (the U.S. patent), and surface areas within the range of from 100 to 800 m.sup.2 /g. These references describe procedures for the preparation of silica gels involving gelling alkali metal silicate solutions with gaseous carbon dioxide or mineral acids, neutralizing either about half (the British specification) of substantially the entire alkali metal silicate content of the hydrogels thus formed, aging the neutralized gels (and, in the case of the U.S. patent, making the gel pH alkaline with ammonium hydroxide), thereafter spray-drying the hydrogel to remove the liquid phase, washing the spray-dried material and re-drying the same for subsequent use. It has, however, been found that these procedures do not enable one to prepare silica gel materials having cumulative pore volumes as large as 2.0 cc/g. Moreover, when silica gels thus made are used as supports for stereospecific catalysts in the particle form polymerization of ethylene, polyethylenes having melt indexes of only up to about 2 are obtained.
From the preceding it will be seen that prior efforts to produce relatively high melt index olefin polymers in particle form operations by the use of modified silica gel catalyst supports and/or other techniques have not been entirely satisfactory. It is therefore a principal object of the present invention to provide an improved silica gel material which, when employed as the support for a stereospecific catalyst utilized in the particle form polymerization of olefins, effects the production of polymers having substantially higher melt indexes than heretofore obtained in such operations. Other objects and advantages of the present invention will be apparent from the following description of the nature and preferred embodiments of the improved silica gel materials hereof.